Drobo 5C ejecting while in use and randomly, 2019 iMac

I bought together a 5C and 5D3 with a 2019 iMac last year. Flawless work flow until two months ago. The 5C randomly ejects now with no apparent pattern. The 5D3 still works great. iMac running OS 10.14.6 original Drobo usb3 and thunderbolt cables connected directly to iMac, work computer - no sleep mode or drive sleep.
Anyone have a similar issue? I’ve noticed a few ejecting forums on here over the years, any light bulbs?